The present application relates to a method for manufacturing an optical disk master to be used for manufacturing an optical disk, a method for manufacturing an optical disk, and an apparatus for manufacturing an optical disk master.
In order to produce a playback-only optical disk in which pit strings are formed from so-called emboss pits, in the production process thereof, first, an optical disk master provided with an uneven pattern corresponding to the pit strings are prepared. Subsequently, a stamper is formed from the optical disk master, and optical disks are mass-manufactured by using the stamper.
In a recordable optical disk (a so-called rewritable disk or write-once disk) which is allowed to record and play back the user data by a phase change recording system or a dye change recording system, grooves constituting a recording track is disposed. In order to produce such an optical disk, in the production process thereof, first, an optical disk master provided with an uneven pattern corresponding to the grooves are prepared. Subsequently, a stamper is formed from the optical disk master, and optical disks are mass-manufactured by using the stamper (refer to International Patent Publication WO 2004/047096 Pamphlet).
With respect to a process for producing an optical disk master, a so-called mastering process, in recent years, a mastering technology referred to as phase transition mastering (PTM) by using an inorganic resist has been known.
In PTM, laser light from a semiconductor laser is applied to a master-forming substrate coated with an inorganic resist and, thereby, exposure is performed by thermal recording.
In the mastering process by using the inorganic resist, individual masters (master-forming substrate provided with an inorganic resist layer) have variations in the recording sensitivity. Consequently, a technique, in which an optimum recording laser power is determined by performing test writing through the use of a region other than an actually used for mastering recording, for example, a region outside or inside the exposure pattern recording region on the master-forming substrate, is adopted so as to deal with the variations in the recording sensitivity of the individual master-forming substrates.
However, the master-forming substrate provided with the inorganic resist layer has not only the individual variation, but also variations in the recording sensitivity in a plane, for example, over the inner perimeter to the outer perimeter, on even one master-forming substrate.
In the case where the resist film is an inorganic thermal recording material, in general, the film is formed by a sputtering method, for example. The difference in film quality of the resist film between the inner perimeter and the outer perimeter occurs depending on the material properties of the target material (mixing ratio, sputtering rate of the constituent materials, and manufacturing method) and characteristics of a sputtering apparatus, for example, chamber shape, exhaust performance (properties), TS distance (distance between target and substrate), magnet shape, magnetic field intensity distribution, degree of vacuum, Ar gas flow rate, and film formation pressure. Even when the film thickness is uniform, it is difficult to maintain the in-plane uniformity of the film quality.
The recording sensitivity and the in-plane uniformity of the sensitivity are also varied delicately in accordance with the usage of the target material in use (the amount of the integral of electric power).
In the master preparation of high density disk, e.g., Blue-ray Disc (registered trade mark, produced by Sony Corporation), the above-described variations in the in-plane recording sensitivity exert an influence on the recording characteristics (precision of the pit and the groove) thereof and cause problems.
Even when the in-plane variations are taken into consideration and the recording sensitivity is checked by performing test writing (test recording) outside the exposure pattern recording region, the sensitivity of the entire area to be used for recording may be estimated roughly. Therefore, when a change occurs in the target life or between the inorganic resist masters, the in-plane uniformity (difference between the inner perimeter and the outer perimeter) of the sensitivity may be varied and the signal characteristics to be exposed and recorded may be varied.
That is, when the recording sensitivity over the inner perimeter to the outer perimeter of the master-forming substrate provided with the inorganic resist layer is not uniform, if recording is performed on the entire signal area with a constant power obtained by the check of the recording sensitivity through the use of the test writing at the inner perimeter portion or the outer perimeter portion, differences in the recorded signal characteristic may occur between the inner perimeter and the outer perimeter.